The present invention relates generally to radio frequency power amplifiers and in particular to a radio frequency power amplifier device for use in a radio frequency power amplifier.
Radio frequency (RF) power amplifier devices for use in wireless transmission applications, such as RF power amplifiers, are typically implemented in an integrated circuit. The integrated circuit includes an RF amplification element, typically an RF transistor such as a Laterally Diffused Metal Oxide Semiconductor (LDMOS), that amplifies an RF signal applied to the device. When an RF power amplifier device is utilized in an RF power amplifier, the RF power amplifier typically further includes an input matching circuit coupled to an input of the RF power amplifier device and an output matching circuit coupled to an output of the RF power amplifier device. The input and output matching circuits are designed to optimize the performance of the RF amplification element included in the RF power amplifier device.
RF transistors such as LDMOS have very low output impedances, making it difficult to design an output matching circuit that will optimize performance of the transistor. In order to facilitate use of such a transistor in an RF power amplifier, RF power amplifier device manufacturers typically incorporate an output matching circuit into the integrated circuit that includes the RF transistor. The output matching circuit is designed to move the output impedance of RF transistor to a different, higher, and more easily matched output impedance at the output of the RF power amplifier device.
Referring now to FIGS. 1 and 2, an exemplary RF power amplifier device of the prior art is illustrated. FIG. 1 is a schematic diagram of RF power amplifier device 100. Typically, RF power amplifier device 100 is implemented in an integrated circuit 200, which integrated circuit is illustrated in FIG. 2. RF power amplifier device 100 includes an RF transistor 106, such as an LDMOS transistor or a Bipolar Junction Transistor (BJT), that amplifies an RF signal applied to the RF power amplifier device. Typically RF transistor 106 is an RF transistor die 206 that is embedded in the integrated circuit. RF power amplifier device 100 further includes an input 101 and an output 120, an input matching circuit 102 coupled between input 101 and an input 105 of RF transistor 106, and an output matching circuit 110 coupled between an output 107 of RF transistor 106 and output 120. Input matching circuit 102 typically comprises an input inductive element 104, such as multiple series input bond wires 202. Each bond wire of the multiple bond wires 202 is coupled to a gate, in the case of an LDMOS transistor, or a base, in the case of a BJT, of RF transistor 106.
Output matching circuit 110 comprises a shunt inductive element 112 and a series inductive element 118 that are each coupled to an output of RF transistor 106. Shunt inductive element 112 typically comprises multiple shunt bond wires 212 that are each coupled to an output of RF transistor die 206, typically a drain of an LDMOS transistor or a collector of a BJT. Series inductive element 118 typically comprises multiple series bond wires 218 that also are each coupled to the output of RF transistor die 206. Ideally, shunt bond wires 212 and series bond wires 218 are coupled to RF transistor die 206 at the die plane in order to minimize any additional, undesired inductances that may result from moving them away from the die plane. Output matching circuit 110 further includes a shunt capacitor 114, such as a metal oxide semiconductor capacitor (MOSCAP) 214, coupled between shunt inductive element 112, that is, shunt bond wires 212, and ground 116. Capacitive element 114 performs a direct current (DC) blocking function and further contributes to the output match provided by output matching circuit 110.
As is well known in the art, an impedance of output circuit 110, and in particular a value of each of elements 112, 114, and 118, presented to RF transistor 106 by each of shunt bond wires 212, capacitor 214, and series bond wires 218, varies with a frequency of the RF signal applied to RF amplifier device 100. As a result, output matching circuit 110 provides an effective output match to RF transistor 106 only over a very limited bandwidth. Since any additional output matching circuitry must be coupled to output 120 of RF power amplifier device 100, the shunt impedances 112, 114 of output matching circuit 110 are isolated from external matching circuitry. In addition, since each bond wire of the multiple shunt bond wires 212 is limited in overall length by a package encasing RF power amplifier device 100, the use of output matching circuit 110, and RF, power amplifier device 100, is limited to high frequency applications, such as frequencies of 2 gigahertz (GHz) and above. As a result, RF power devices that include internal matching circuitry are typically narrow band devices that are limited to high frequency applications.
Therefore, there is a need for an RF amplifier device that can be used across a wide band of frequencies and that can be designed into low frequency, as well as high frequency, applications.